PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Benefit Obligation Rollforward (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Lump-sum payments recognized as settlements			$ 71.1	$ 22.1
Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year			692.4	503.8
Service cost			17.3	10.2		$ 9.2
Interest cost			29.5	24.5		17.0
Actuarial (gain) loss			(12.8)	39.4
Settlement loss			16.5	1.2
Gross benefits paid			(90.6)	(36.9)
Gross benefits paid			(19.5)	(36.9)
Benefit obligation, end of year	$ 692.4	$ 503.8	652.3	692.4		503.8
Accumulated benefit obligation, end of year	613.7		591.4	613.7
Lump-sum payments recognized as settlements			71.1	22.1		79.5
Pension Plans | Energen
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Energen divestiture			0.0	0.0
Pension Plans | Alagasco
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Alagasco acquisition			0.0	150.2
Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year			258.5	180.1
Service cost			12.8	11.3		10.2
Interest cost			11.2	8.9		5.2
Actuarial (gain) loss			(23.7)	1.2
Plan amendments			(4.9)	2.5
Retiree drug subsidy program			0.4	0.0
Gross benefits paid			(15.1)	(7.3)
Benefit obligation, end of year	258.5	180.1	239.2	258.5		180.1
Postretirement Plans | Energen
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Energen divestiture			0.0	0.0
Postretirement Plans | Alagasco
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Alagasco acquisition			0.0	61.8
Laclede Gas
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Lump-sum payments recognized as settlements			58.2
Laclede Gas | Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year			543.6	503.8
Service cost			11.5	9.7		9.2
Interest cost			23.3	24.0		17.0
Actuarial (gain) loss			(20.7)	41.5
Settlement loss			14.5	1.2
Gross benefits paid			(74.6)	(36.6)
Gross benefits paid			(16.4)	(36.6)
Benefit obligation, end of year	543.6	503.8	497.6	543.6		503.8
Accumulated benefit obligation, end of year	484.1		456.9	484.1
Lump-sum payments recognized as settlements			58.2
Laclede Gas | Pension Plans | Energen
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Energen divestiture			0.0	0.0
Laclede Gas | Pension Plans | Alagasco
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Alagasco acquisition			0.0	0.0
Laclede Gas | Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year			197.9	180.1
Service cost			12.3	11.2		10.2
Interest cost			8.6	8.7		5.2
Actuarial (gain) loss			(10.9)	2.2
Plan amendments			(4.9)	2.5
Retiree drug subsidy program			0.0	0.0
Gross benefits paid			(11.1)	(6.8)
Benefit obligation, end of year	197.9	180.1	191.9	197.9		$ 180.1
Laclede Gas | Postretirement Plans | Energen
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Energen divestiture			0.0	0.0
Laclede Gas | Postretirement Plans | Alagasco
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Alagasco acquisition			0.0	0.0
Alagasco
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Lump-sum payments recognized as settlements			12.9
Alagasco | Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	293.4		148.8
Service cost	5.1	14.2	5.8		$ 14.2
Interest cost	4.1	$ 11.2	6.2		11.2
Actuarial (gain) loss	7.8		7.9
Settlement loss	0.0		2.0
Gross benefits paid	(33.8)		(16.0)
Gross benefits paid	(33.8)		(3.1)
Benefit obligation, end of year	148.8		154.7	148.8	293.4
Accumulated benefit obligation, end of year	129.6		134.5	129.6
Lump-sum payments recognized as settlements			12.9
Alagasco | Pension Plans | Energen
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Energen divestiture	(127.8)		0.0
Alagasco | Pension Plans | Alagasco
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Alagasco acquisition	0.0		0.0
Alagasco | Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	63.3		60.6
Service cost	0.4		0.5		1.7
Interest cost	1.9		2.6		3.5
Actuarial (gain) loss	4.3		(12.8)
Plan amendments	0.0		0.0
Retiree drug subsidy program	0.3		0.4
Gross benefits paid	(4.0)		(4.0)
Benefit obligation, end of year	60.6		47.3	$ 60.6	$ 63.3
Alagasco | Postretirement Plans | Energen
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Energen divestiture	(5.6)		0.0
Alagasco | Postretirement Plans | Alagasco
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Alagasco acquisition	$ 0.0		$ 0.0